“The basic working principle of this system is that the integrated chip MAX038 outputs a high-frequency signal, and the multivibrator composed of NE555 obtains a low-frequency signal through a two-frequency circuit composed of 7474 and a low-pass filter composed of MAX029 and peripheral circuits. Then modulate the high frequency signal with the low frequency signal to obtain the ultrasonic wave of the appropriate frequency. Since the signal is relatively weak, a power amplifier is needed to amplify the signal. The power amplifier circuit here adopts a three-stage amplifier circuit, which is then output through the transducer, and the output intensity of the ultrasonic wave is controlled by the single-chip microcomputer.
At present, most ultrasonic therapeutic apparatuses (physiotherapy type) at home and abroad generally work in the form of continuous sine wave (1-3 MHz) or pulsed sine wave (low frequency modulation about 100 kHz), and the output sound intensity is ≤ 3.0 W /cm2, the existing problem is that the ultrasound mainly acts on the skin, muscle and connective tissue, the peak value of the emission waveform is high, and the penetrating power is poor, so the therapeutic effect on the deeper lesions is not obvious. Therefore, it takes a long time to treat and promote muscle recovery and bone healing. The designed physiotherapy device can generate specific ultrasonic energy with low waveform peak and strong penetrating power, which can act on the musculoskeletal deeper and accelerate the healing. This design uses the design idea of circuit modularization, and combines with reality a simple and applicable multi-parameter ultrasonic therapeutic apparatus. At present, there is no similar product report on this design at home and abroad, and the product design principle involves many research results and reports on the cutting-edge ultrasonic treatment of bone injury.
1 System composition principle
The system is mainly divided into 5 main modules. Among them, the single-chip control module is the system, which controls the intensity of ultrasonic waves. Others (such as MAX038) mainly generate high-frequency waves. NE555 constitutes a multivibrator. The ultrasonic waves generated after mixing can be amplified and output to act on the injured part of the human body. The basic system structure is shown in Figure 1.
The basic working principle of this system is that the integrated chip MAX038 outputs a high-frequency signal, and the multivibrator composed of NE555 obtains a low-frequency signal through a two-frequency circuit composed of 7474 and a low-pass filter composed of MAX029 and peripheral circuits. Then modulate the high frequency signal with the low frequency signal to obtain the ultrasonic wave of the appropriate frequency. Since the signal is relatively weak, a power amplifier is needed to amplify the signal. The power amplifier circuit here adopts a three-stage amplifier circuit, which is then output through the transducer, and the output intensity of the ultrasonic wave is controlled by the single-chip microcomputer.
2 Generation of high frequency signals
The MAX038 high-frequency waveform generator chip of Maxim’s can be used to generate high-frequency signals. Various waveform curves of this method can be represented by trigonometric function equations. A function signal capable of generating various waveforms. Function signal generators can also be made of general-purpose devices such as transistors and op-amp ICs. Maxim’s function signal generator MAX038 has high frequency and good precision. Therefore, it is called a high-frequency precision function signal generator IC. The high-frequency signal generation circuit composed of the chip is shown in Figure 2.
In Figure 2, the 3rd and 4th pins (ie A0 and A1 pins) are the control output waveforms, and the output waveform frequency is determined by the external capacitor C1 and the 8th and 10th pins.
3 Low-frequency signal generation circuit
The low-frequency signal generation module circuit can use NE555 to form a multivibrator. When the oscillator outputs a waveform signal, it is connected to a 7474 latch as the main component to form a two-frequency circuit, and then MAX298 is used to form a low-pass filter for filtering. , the desired low-frequency signal wave can be obtained. The MAX29X series 8th-order low-pass switched capacitor filter produced by MAXIM company is easy to use, simple in design and small in size (with 8-pin DIP package). a wide range of applications. The reason why MAX03 8 is not used here to generate low-frequency waves is that when the DC levels of the two waveforms are different, the effect is not obvious after mixing. And use MAX298 to filter, you can get a very good filtering effect within the frequency of 5 kHz to 10 kHz. The specific low-frequency signal generating circuit is shown in Figure 3. The oscillation period is determined by R1, R2 and C1 in NE555, and the specific formula is:
4 Design of the mixer circuit
Mixing can be implemented with a multiplying circuit, and the circuit can be designed with the following relationship:
Because the multiplication circuit can easily realize the multiplication of two analog signals, and, with the multiplication circuit as the basic unit, it can also easily form operation circuits such as division, power and square root. Therefore, in the field of radio communication, modulation and demodulation circuits can also be formed using multiplication circuits. At present, there are already multiplication operations on the market, which are called analog multipliers.
The expression for an analog multiplier is:
In the formula, k is the proportional coefficient, and the proportional coefficient of each circuit is generally not the same.
In the design of this circuit, the multiplier is mainly used to realize the multiplication of two signals to change the low frequency signal to the intermediate frequency. This design requires two frequency signals, one is the carrier wave (ie high frequency signal), and the other is the modulated wave (ie low frequency signal). To achieve AM modulation, a reasonable multiplier circuit needs to be used. This design is selected by Motorola’s MC1496 integrated double-balanced analog multiplier. MC1496 is a high-performance analog multiplier produced by Motorola. It can be used as a broadband, suppressed carrier double-sideband balanced modulator. It does not require a coupling transformer or tuning circuit. It can also be used as a high-performance SSB multiplying detector and AM solution. Modulator / demodulator, FM demodulator, mixer, frequency multiplier, phase detector, etc.
MC1496 can use single power supply or dual power supply, both of which have the same effect. When powered by a single power supply, its DC bias is realized by external components. This design selects a single power supply, and the single power supply output is a ground signal. In dual power supplies, the output is a dual output differential signal. Its specific circuit is shown in Figure 4.
5 Power Amplifier Design
In addition to general circuits such as rectification, oscillation, amplification and protection in general amplifiers, the power amplifier circuit of the ultrasonic generator should also have some special circuits, such as automatic frequency tracking, matching, automatic power control, vibration system amplitude control, etc. circuit. These circuits are required by the special nature of the generator load (ie the transducer). Usually the generator has a load value (sometimes also called output impedance), only when the actual load is equal to this value, the generator can work in the design state and provide the rated output to the load. Otherwise, impedance transformation is required through the output transformer. In addition to this, tuning must be done, since the transducer is a resistive load, and therefore, a direct connection to the generator produces considerable reactive losses, which not only reduces efficiency, but also affects the generator safe use. Therefore, an opposite reactance element is required to “cancel” the effects of the reactance component in the transducer, which is called tuning. In this power amplifier, the most important thing is matching. Since the piezoelectric transducer has electrostatic capacitance, and the magnetostrictive transducer has electrostatic induction, when the transducer is in resonance state, the voltage and current on the transducer There is a phase angle φ between them, and the output power P=cosφ. Due to the existence of φ, the output power cannot reach the value. Only when φ=0, the output power can reach the value. Therefore, in order to make the voltage VRL on the transducer be in phase with the current (φ=0), it is necessary to connect a canceling impedance in parallel or in series on the transducer. For piezoelectric transducers, it is sufficient to connect an Inductor in parallel or in series, while for magnetostrictive transducers, a capacitor should be connected in parallel or in series.
For the power amplifying part of the ultrasonic wave, some measurements and calculations are made in this paper, and the actual circuit is designed for three-stage amplification. Figure 5 shows its front-end circuit, in which a field effect transistor is used as the front-end amplification of the stage, and then the emitter is used to amplify the current. Each amplifier has added an emitter-collector output circuit at the output to reduce the output impedance, so as to achieve the effect of amplifying the current. The intermediate stage triode amplifying circuit is shown in Figure 6, its input and output have capacitive filtering, and the output follower is added to reduce the output impedance. The final stage triode amplifying circuit adopts the 2SC3281 power triode of Toshiba Company, and its circuit is shown in Figure 7.
6 MCU control
This design uses the AT89C2051 microcontroller of ATMEL Company to form the control system. Because the requirements of the system are not very strict, that is, it does not require high computing speed, nor does it require large memory, but only requires a certain degree of shock resistance, and the price is cheap, so as to achieve the purpose of economical benefits. Therefore, the author chooses AT89C2051 CPU. The operating temperature range of this CPU is -40℃～125℃, and it has 20 pins. It is a simplified version of the 8051 type CPU, which fully meets the use requirements of this system. Since the main program mainly completes the module initialization and the calling of its own program, its structure is clear and simple. Figure 8 shows its control software flow chart.
The second is the initialization module, whose function is to restore the timer, interrupt, etc. to the original state. For example, make the timer work in working mode 1, interrupt priority of timer 1 and so on. Its initialization flow chart is shown in Figure 9.
The third is power output adjustment. The output power of the physiotherapy device is divided into three levels (low, medium and high). In the design, the output of the single-chip microcomputer is used to control the static bias voltage of the subsequent BJT circuit, so as to change the static operating point of the BJT and make the piezoelectric crystal The amplitude of the output voltage across both ends changes to regulate the output power. This function can be achieved by calling the interrupt service routine through an external key interrupt during design. The principle is to judge which interrupt to enter by the number of keystrokes, and to call the corresponding subroutine. Since the power here is divided into three grades, the remainder can be judged by dividing the number of keys by 4, namely:
If the number of keys is 4N (N=0, 1, 2, 3…), then no power is output;
If the number of keys is 4N+1 (N=0, 1, 2, 3…), the low power output subroutine is called;
By analogy, three different power outputs can be achieved.
Here is part of its code:
This design mainly gives a more detailed realization scheme of ultrasonic wave from generation to control. Compared with other methods, this scheme is relatively simple, and the cost is low, and the devices used are relatively common. The high-frequency and low-frequency signals are generated independently, which is easy to adjust the two signals, and the single-chip microcomputer is used as the one, which has high flexibility, and can also be modified according to different requirements, so as to meet various actual needs.
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